Origin of glass and its relationships with phlogopite in mantle xenoliths from central Sardinia (Italy)

Spinel-bearing lherzolite and harzburgite mantle xenoliths from central Sardinia (Italy) contain glassy patches with clinopyroxene and spinel relicts together with euhedral crystallites of olivine, clinopyroxene and spinel. Although texturally uniform, the glass may be chemically distinguished into two groups: the first type with low K2O (mean 0.07 ± 0.05 wt%) and TiO2 (mean 0.68 ± 0.12 wt%; LKT-type), and the second type with high K2O (mean 2.51 ± 0.82 wt%) and TiO2 (mean 3.14 ± 0.35 wt%; HKT-type). All glass compositions are quartz-normative. Rare phlogopite laths are enclosed in olivine or orthopyroxene crystals. The following mass balance equations have been obtained both for HKT- and LKT-type glasses: {equation presented} and {equation presented} where the subscripts (1) and (2) represent primary (reactant) and secondary (product) phases, respectively. Clinopyroxene + spinel ± phlogopite are expected to have melted incongruently to yield olivine, clinopyroxene and spinel. This suggests that the glasses are not the product of metasomatic processes, but derive by decompression melting of mantle xenoliths, probably after the incorporation in the host lava. Other evidences of early stages of partial melting come from the relatively abundant spongy-textured clinopyroxene. Recent experimental studies (Raterron et al., 1997) evidence the incongruent partial melting of clinopyroxene at mantle depth several hundreds degrees below its conventional solidus temperature. Metasomatic processes in the Sardinian lithospheric mantle account for the origin of phlogopite in the xenoliths.